Responses of net ecosystem carbon exchange to diffuse radiation in an alpine meadow on the Qinghai-Tibetan Plateau, China.

Abstract

Qinghai-Tibetan Plateau, one of the regions on the earth that receives the most solar radiation, is the world's highest alpine meadow ecosystem, with significance to regional and global carbon cycles. To examine the effects of solar radiation on ecosystem carbon dynamics in an alpine meadow, the net ecosystem CO2 exchange (NEE), solar radiation, diffuse radiation, and related environmental variables were measured using eddy-covariance technique and micro-meteorological system. Sky conditions were divided into three categories of clear days (CI≥0.7), cloudy days (0.3<CI<0.7) and overcast days (CI≤0.3) by clearness index (CI, defined as the ratio of solar radiation observed at meadow to the top of the earth's atmosphere). We analyzed the changes in NEE and the effects of related factors on it under different CI. The results showed that the maximum NEE (-0.63 mg CO2·m-2·s-1) appeared in photosynthetic photon flux density (PPFD) of approximately 1400 μmol·m-2·s-1, corresponding the CI of 0.6-0.7 in cloudy days, which was higher than the maximum NEE in clear days (-0.57 mg CO2·m-2·s-1) (negative and positive NEE represented carbon uptake and emission, respectively, here absolute value was used to describe NEE). With the increases of diffuse radiation, NEE increased logarithmically when CI<0.6, reached the highest value when CI ranged from 0.6 to 0.7, and then decreased when CI≥0.7, indicating that photo-inhibition might occur under high solar radiation conditions and increasing diffuse radiation would improve carbon sequestration of alpine meadow. Ecosystem respiration (Re) increased exponentially with the increases of air temperature (Ta). The maximum NEE was found at the air temperature of about 15 ℃ from the relationship of NEE and air temperature, but NEE tended to decrease with increasing air temperature when Ta>15 ℃. Under clear sky day conditions, Re increased with increasing CI due to the increases of air temperature, with negative effects on NEE. NEE increased with the increases of VPD up to 0.6 kPa, then slowly decreased when VPD>0.6 kPa, illustrating that NEE was reduced due to the relatively high VPD. Our results suggested that strong solar radiation on clear days would not increase carbon uptake capacity of alpine meadow, while cloudy days with clearness index of 0.6-0.7 would help increase carbon sequestration on the Qinghai-Tibetan Plateau.